(E  ORIGIN  OF  VERTEBRATE  LIMBS 


RECENT  EVIDENCE  UPON  THIS  PROBLEM  FROM 
STUDIES  ON  PRIMITIVE  SHARKS. 


RAYMOND  C.  OSBURN. 


itted  in  Partial  Fulfilment  of  the  Requirements  for 
ie  Degree  of  Doctor  of  Philosophy,  in  the  Faculty 
of  Pure  Science,  Columbia  University. 


Reprinted  from  the 

ai.s  of  the  New  York  Academy-  of  Sciences,  Vol.  XVII,  Part  II. 


May  21,  1906, 


THE  ORIGIN  OF  THE  VERTEBRATE  LIMBS. 


RECENT  EVIDENCE  UPON  THIS  PROBLEM  FROM 

Studies  On  Primitive  Sharks. 


BY 

RAYMOND  C.  OSBURN. 


Submitted  in  Partial  Fulfilment  of  the  Requirements  for 
the  Degree  of  Doctor  of  Philosophy,  in  the  Faculty 
of  Pure  Science,  Columbia  University. 


Reprinted  from  the 

Annals  of  the  New  York  Academy  of  Sciences,  Vol.  XVII,  Part  II. 


May  20,  1906. 


^-|3 

Os\ 


[Annals  N.  Y.  Acad.  Sci.  Vol.,  XVII,  No.  2,  Part  II,  May  ao; 


1906.] 


THE  ORIGIN  OF  VERTEBRATE  LIMBS. 

RECENT  EVIDENCE  UPON  THIS  PROBLEM  FROM  STUDIES  ON  PRIM- 
ITIVE SHARKS. 

By  Raymond  C.  Osburn. 

For  nearly  thirty  years  the  Fin-fold  Theory  has  been  commonly 
accepted  to  explain  the  origin  of  the  limbs  of  the  Vertebrata. 
This  theory  has  as  its  main  thesis  that  all  fins,  both  paired  and 
unpaired,  have  arisen  in  situ  and  in  the  same  manner,  as  local 
developments  from  the  body  wall.  Thus  conceived,  they  are 
primarily  external  structures  which  have  as  their  primitive  form 
a longitudinal  fold  of  skin  supplied  with  muscles,  nerves,  and 
blood-vessels  derived  in  a segmental  way  from  the  adjoining 
body  wall,  and  with  supporting  structures  which  have  had  their 
origin  within  the  fins.  This  theory  first  took  form  in  the  work 
of  Thacher  (’ 77),  Mivart  (’79),  and  Balfour  (’78),  all  of  whom 
arrived  at  the  same  conclusion  independently,  the  first  two  on 
anatomical  grounds,  the  last  from  the  embryological  standpoint. 
The  theory  has  been  ably  supported  by  Dohrn  (’83  and  ’02), 
Paul  Mayer  (’86),  Wiedersheim  (’92),  Mollier  (’93),  Rabl  (01), 
Dean  (’02),  Regan  (’04),  and  others. 

Opposed  to  this  view  is  the  older  “ Archipterygium  ” or  Gill- 
arch  Theory,  first  definitely  stated  by  Gegenbaur  (’65  and  ’70) 
and  maintained  by  him  through  all  his  later  work  (’95) . Ranged 
on  this  side  of  the  question  are  Bunge  (’74),  von  Davidoff  (’79 
and  ’80),  Furbringer  (’96  and  ’02),  Braus  (’98  and  ’04),  and  others 
of  the  Gegenbaur  school.  As  far  as  the  origin  of  the  unpaired 
fins  is  concerned,  the  gill-arch  theorists  admit  that  they  arose 
as  local  outgrowths,  and  go  farther  than  their  opponents  in 
assuming  a rigid  metamerism  of  all  the  structures  of  the  unpaired 
fin,  deriving  the  median  fin  skeleton  from  processes  (dorsal  and 

1 


P 16019 


2 


OSBURN 


haemal  spines)  of  the  axial  skeleton,  while  according  to  the 
fin-fold  theory  the  skeleton  is  supposed  to  have  been  developed 
independently  of  the  vertebral  column.  But  it  is  in  the  origin 
of  the  paired  fins  that  differences  of  opinion  are  most  in  evidence, 
for  while  the  fin-fold  theorists  consider  that  the  paired  fins  have 
had  in  a general  way  a similar  origin  to  the  unpaired,  the  gill- 
arch  theorists  hold  that  they  have  been  modified  from  gills 
and  that  the  girdles  and  rays  of  the  fins  are  directly  homologous 
with  the  supporting  structures  (arches  and  rays)  of  the  gills. 

During  all  the  years  in  which  these  theories  have  been  under 
discussion  unsettled  points  have  not  been  lacking,  and  within 
recent  time  a number  of  objections  have  been  urged  against  the 
fin-fold  theory.  With  the  hope  of  deciding  some  of  these  vexed 
questions  and  with  a view  to  testing  the  validity  of  the  objec- 
tions raised,  the  writer  has  been  led  to  investigate  the  develop- 
ment of  the  paired  and  unpaired  fins  of  a cestraciont  shark 
(Heterodontus  japonicus,  Dumeril),  a form  belonging  by  direct 
lineage  to  a group  of  very  ancient  sharks, — much  older,  as  far 
at  least  as  can  be  judged  by  palaeontological  data,  than  any 
selachian  that  has  hitherto  been  investigated.1  For  comparison 
the  writer  has  had  various  stages  of  Spinax,  Mustelus,  and 
Torpedo,  and  has  been  especially  fortunate  in  having  access  to 
a number  of  embryonic  stages  of  Chlamydoselachus  anguineus 
Garman,  a form  generally  recognized  as  being  one  of  the  most 
primitive  of  modern  Selachii.  The  series  of  Cestracion  (Hetero- 
dontus) embryos  at  my  command  is  very  complete.  For  the  use 
of  all  of  this  valuable  material  my  thanks  are  due  to  Professor 
Bashford  Dean,  to  whom  I am  also  grateful  for  much  encourage- 
ment and  many  helpful  suggestions  in  pursuit  of  the  work. 

The  present  paper  embodies  only  the  main  results  of  my 
studies  and  will  be  followed  by  another  more  extended  in  scope, 
in  which  will  be  given  the  evidence  upon  which  these  results 
are  based  and  in  which  the  literature  of  the  subject  will  be  treated. 

It  may  be  briefly  stated  that  the  results  of  my  work  indicate 
that  many  of  the  objections  raised  against  the  fin-fold  origin 
of  the  paired  fins  apply  equally  well  to  the  unpaired  fins,  which 

1 All  references  to  Cestracion,  Chlamydoselachus,  and  Spinax  in  the 
following  pages  are  from  my  own  observations,  unless  otherwise  accredited. 


THE  ORIGIN  OF  VERTEBRATE  LIMBS 


3 


iare  held,  even  by  those  who  have  raised  the  objections,  to  be 
of  strictly  metameric  and  local  origin.  Other  objections  can 
be  shown  to  be  based  upon  faulty  evidence,  or  upon  facts  which 
bear  a different  and  more  probable  interpretation. 

The  principal  objections  to  the  fin-fold  theory  are,  briefly 
stated,  as  follows: 

A.  Relating  to  the  comparison  of  the  paired  fin  girdles  with 
gill  arches  the  followers  of  Gegenbaur  contend  that: 

I.  The  pectoral  girdle  arises  in  serial  order  with  the  gill 
arches. 

II.  The  pectoral  girdle  makes  its  appearance  earlier  than  the 
basalia  and  rays. 

III.  The  basalia  grow  out  of  the  girdles  and  the  rays  out  of 
the  basalia. 

IV.  The  dorsal  end  of  the  pectoral  arch  comes  into  relation 
with  the  visceral  muscles. 

V.  The  pelvic  girdle  is  the  homolog  of  the  pectoral  in  every 
respect  and  so  corresponds  to  a gill  arch. 

B.  Relating  to  the  supposed  migration  of  the  paired  fins 
from  the  gill  region: 

I.  The  pelvic  fins  have  been  shown  to  undergo  slight  migra- 
tion or  shifting  during  ontogeny. 

II.  As  a proof  of  migration  there  appears  a strong  collector 
nerve  in  the  anterior  part  of  the  pelvic  fins. 

III.  Also,  in  the  early  development  of  the  pelvic  fin  the  most 
anterior  muscle-buds  degenerate  without  entering  the  fin,  while 
those  just  posterior  to  these  are  compelled  to  reach  backward 
to  enter  the  fin. 

C.  Relating  to  the  contrast  of  paired  with  unpaired  fins: 

I.  The  skeleton  of  the  unpaired  fins  consists  of  modified 
vertebral  (spinous  and  haemal)  processes. 

II.  The  girdles  of  the  paired  fins  have  nothing  to  represent 
them  in  the  unpaired  fins. 

III.  The  presence  of  post-axial  rays  in  the  pectoral  fin  proves 
the  primitive  biseriality  of  the  paired  fins. 

IV.  The  fin-rays  of  the  paired  fins  do  not  arise  separately 
and  later  become  fused  to  form  basalia  as  the  fin-fold  theory 
would  lead  us  to  expect. 

V.  The  fusion  of  the  muscle-buds  in  the  paired  fins  before 


4 


OSBURN 


the  appearance  of  the  skeleton  precludes  the  possibility  of  the 
metameric  origin  of  the  latter. 

VI.  The  early  discrepancy  between  muscles  and  rays  in  the 
pelvic  fin  proves  the  primitive  dysmetamery  and  independent 
origin  of  the  paired  fin  skeleton. 

Let  us  now  examine  the  foregoing  objections  point  by  point 
and  determine  whether  they  are  well  founded. 

A. 

I.  The  pectoral  girdle  cannot  be  considered  serially  homolo- 
gous with  the  gill  arches  for  the  following  reasons: 

(1)  The  first  anlage  of  the  pectoral  girdle  lies  almost  its 
whole  length  below  the  gill  arches,  as  shown  by  Braus’s  recon- 
structions (’04)  of  Spinax,  and  by  my  own  observations  on 
Cestracion. 

(2)  It  arises  near  the  external  wall  of  the  body,  while  the  gill 
arches  arise  near  the  pharyngeal  wall,  according  to  my  studies 
on  Cestracion. 

(3)  The  thickening  of  mesenchyme  from  which  the  girdle  is 
differentiated  takes  its  origin  next  to  the  ectoderm  and  spreads 
in  an  inward  direction  till  it  occupies  all  the  region  in  which  the 
pectoral  fin  skeleton  arises,  which  latter  is  therefore  of  external 
origin,  since  it  arises  out  of  this  mesenchyme  thickening.  The 
gill  arches,  on  the  other  hand,  arise  next  to  the  enteron. 

(4)  The  study  of  Cestracion  shows  that  the  first  anlage  of  the 
pectoral  fin  lies  wholly  within  the  region  of  spinal  muscles. 

(5)  In  Cestracion  at  least,  the  pectoral  girdle  is  relatively 
much  farther  from  the  last  gill  arch  at  its  first  appearance  than 
it  is  during  later  growth.  That  is,  it  grows  toward  the  gill -arch 
region. 

(6)  The  first  four  points  apply  with  even  more  force  to  the 
pelvic  girdle  which  in  a general  way  must  be  considered  the 
homolog  of  the  pectoral. 

II.  The  observations  of  E.  Ruge  (’02)  and  Braus  (’04)  that 
the  pectoral  girdle  of  Spinax  niger  is  the  first  part  of  the  fin 
skeleton  to  make  its  appearance,  are  probably  not  to  be  ques- 
tioned, but  since  Spinax  is  the  only  form  so  far  examined  in 
which  the  time  relations  are  thus,  and  as  in  Cestracion,  which 
is  unquestionably  a much  older  type  of  fish,  the  first  anlage 


THE  ORIGIN  OF  VERTEBRATE  LIMBS 


5 


of  the  skeleton  undoubtedly  makes  its  appearance  in  the  region 
of  the  base  of  the  rays  and  the  neighboring  part  of  the  primary 
basal,  we  must  conclude  that  this  objection  is  at  least  not  final. 
On  the  contrary,  it  seems  plain  from  the  evidence  at  hand  that 
Spinax  is  the  exception  to  the  rule. 

III.  Against  this  objection  we  must  weigh  the  following  facts: 

(1)  The  writer  finds  that  in  Cestracion  the  rays  and  basalia 
begin  to  appear  before  the  girdle.  Other  investigators  since 
the  time  of  Balfour  (’78)  have  found  the  same  to  be  true  in 
various  species  (Spinax  excepted). 

(2)  The  basalia  and  rays  do  not  “grow”  out  of  any  pre- 
existing structure,  but  are  differentiated,  both  in  the  same 
manner,  out  of  the  same  band  or  layer  of  mesenchyme.  This 
layer  gives  rise  also  to  the  rudiment  of  the  girdle.  The  above 
distinction  is  an  important  one  since  upon  it  depends  the 
interpretation  that  all  of  these  skeletal  structures  of  the  fins 
are  developed  in  situ  by  differentiation  instead  of  “growing 
out.  ” The  latter  term  implies  internal  development  and  change 
of  location  during  the  process,  a condition  contrary  at  least  to 
the  present  observations. 1 

IV.  The  fact  that  the  dorsal  end  of  the  pectoral  girdle  has 
relations  with  the  visceral  musculature  (trapezius  group,  in- 
nervated by  viscero-motor  nerves) , has  been  interpreted  by 
the  gill-arch  theorists  to  show  a primitive  connection, — “die 
alten  Relikte  der  einstmaligen  kopfmuskelversorgung  des  Schult- 
ergurtels’’  (Furbringer  ’02).  The  connection  is  beyond  question, 
but  that  the  above  conclusion  is  not  final  is  evident  from  the 
fact  that  in  Cestracion  the  first  anlage  of  the  pectoral  girdle 
arises  entirely  ventral  to  the  anlage  of  the  trapezius,  quite 
separated  from  it,  and  comes  into  relation  with  the  visceral 
muscles  only  by  its  later  dorsal  growth.  This  is  true  also  of 
Spinax,  as  shown  by  my  preparations  of  a 20  mm.  embryo. 

V.  The  pelvic  girdle  cannot  be  strictly  homologized  with 
the  pectoral,  point  for  point,  for  the  following  reasons: 

(1)  In  the  oldest  fossil  sharks  in  which  the  pelvic  is  sufficiently 

1 By  way  of  comparison,  it  is  worthy  of  notice  that  the  muscle-buds  do 
grow  into  the  fins.  They  disarrange  the  mesenchyme  cells  and  push 
them  out  of  the  way  during  their  progress.  Nothing  of  this  nature 
takes  place  in  connection  with  the  development  of  the  fin  skeleton. 


6 


OSBURN 


known  (Pleuroptervgidae)  there  is  no  pelvic  girdle  developed 
beyond  the  condition  of  basalia.  If  the  gill-arch  theory  were 
true,  the  pelvic  girdle  should  be  best  developed  in  the  oldest 
forms. 

(2)  In  the  lowest  modern  sharks  (Notidanidae)  there  is  in 
the  adult  no  evidence  of  a dorsal  prominence  in  the  pelvic  to 
correspond  with  the  scapular  portion  of  the  pectoral.  It  is 
difficult  to  see  how  the  pelvic  girdle  of  Chlamydoselachus,  a 
long,  flat  plate  pierced  with  eight  nerve  foramina,  could  be 
made  to  homologize  with  the  pectoral  girdle  of  any  shark. 

(3)  In  none  of  the  stages  of  Chlamydoselachus  in  my  pos- 
session in  which  the  pelvic  is  sufficiently  developed  (from  no 
mm.  upward)  is  there  any  indication  of  a dorsal  prominence. 
The  pelvic  girdle  develops  as  a flat  basale-like  plate. 

(4)  The  argument  for  the  anterior  prominence  which  is 
present  in  some  sharks  and  which  was  originally  homologized 
with  the  scapula  by  von  Davidoff  has  already  been  given  up  by 
the  gill-arch  theorists.  The  small  dorsal  prominence  recently 
described  (Braus  ’04)  in  the  pelvic  of  Spinax  and  homologized 
with  the  scapula  can  scarcely  be  considered  homologous,  for 
the  reason  that  it  is  situated  posterior  to  the  nerve  foramen, 
while  the  scapular  portion  of  the  pectoral  girdle  is  always,  so 
far  as  my  observations  have  extended,  anterior  to  the  foramen. 

The  various  parts  of  the  pelvic  girdle  cannot,  therefore,  be 
homologized  with  all  parts  of  the  pectoral,  and  certainly  the 
pelvic  girdle  is  much  farther  removed  from  comparison  with 
the  gill  arch. 

B.  It  becomes  evident  that  no  migration  yet  shown  is  quan- 
titatively sufficient  to  account  for  the  distance  between  the 
pelvic  fin  and  the  gill  region,  for: 

I.  The  slight  measurable  migration  which  the  pelvic  may 
undergo  in  its  ontogeny  cannot  be  accepted  as  evidence  in  favor 
of  migration  from  the  branchial  region. 

(1)  The  pelvic  fin  in  some  cases  migrates  forward  during 
ontogeny. 

(2)  The  supposed  demonstration  of  migration  in  the  pelvic 
fin  at  an  early  period  is  better  explained  as  due  to  the  concen- 
tration of  the  fin  from  a longer  basis,  as  I will  show  under  another 
heading. 


THE  ORIGIN  OF  VERTEBRATE  LIMBS 


7 


(3)  The  migration  of  the  pelvic,  as  well  as  of  other  fins, 
has  been  shown  biometrically  to  occur  in  accord  with  the  shifting 
of  the  center  of  gravity  during  development  (in  Cestracion  by 
Dean  ’02).  By  the  same  method  it  has  been  proved  that  the 
pelvic  shifts  its  position  in  correlation  with  the  dorsals  (in 
Spinax  by  Punnett  ’04) . Hence  the  observed  migration  becomes 
merely  an  adaptive  process  without  any  special  meaning  in 
phylogeny. 

(4)  While  we  have  no  direct  evidence  that  any  fin  has  ever 
migrated  backward  to  any  extent,  we  have  abundant  proof  of 
the  migration  of  the  pelvic  fins  forward  in  many  Teleosts,  in 
extreme  cases  to  a position  in  front  of  the  pectorals.  This, 
also,  can  be  due  only  to  adaptation. 

(5)  No  satisfactory  reason  has  ever  been  offered  why  a fin, 
when  once  in  the  most  important  place  in  the  body,  viz.  the 
pectoral  position,  should  ever  have  migrated  out  of  it  into  a 
region  of  such  minor  importance  as  the  pelvic  fin  occupies  in 
sharks  and  other  primitive  fishes. 

II.  The  argument  based  upon  the  collector  nerve  is  negatived 
at  once  by  the  following  facts: 

(1)  Collector  nerves  appear  also  in  the  unpaired  fins,  both 
in  the  anterior  and  posterior  parts  of  the  fins,  as  Paul  Mayer 
(’86)  showed  in  Acanthias,  Heptanchus,  and  Centrophorus,  and 
as  my  own  observations  show  in  the  first  and  second  dorsals 
of  Cestracion.  Yet  it  is  contrary  to  the  Gegenbaurian  con- 
ception of  the  unpaired  fins  that  they  have  migrated  at  all  in 
their  phylogeny. 

(2)  A small  posterior  collector  is  known  in  the  pelvic  fins  of 
certain  species.  This  is  assumed  by  the  gill-arch  theorists  to 
be  due  to  a secondary  migration  of  the  fin  in  a forward  direction. 
However,  the  results  of  Punnett’s  studies  (’oo)  on  Mustelus 
indicate  that  in  this  species  the  pelvic  fin  of  the  female  migrates 
farther  forward  than  that  of  the  male  and  yet  has  no  posterior 
collector,  while  that  of  the  male  which  indicates  less  migration 
none  the  less  possesses  it.  ' 

(3)  In  those  Teleosts  which  show  a migration  of  the  pelvic 
to  a thoracic  position,  the  nerves  of  the  fin  are  carried  forward 
during  the  process,  while  no  nerves  are  picked  up  on  the  way 
and  no  new  collector  is  formed  as  a result  of  the  migration. 


8 


OSBURN 


The  hypothesis  of  migration  is  therefore  very  far  from  meet- 
ing all  the  conditions.  The  only  adequate  explanation  (Mollier 
’93)  is  that  all  the  fins  have  shortened  up  at  the  base  and  have 
formed  the  collectors  by  bringing  together  nerves  which  once 
entered  separately  to  innervate  the  longer  fin.  In  favor  of  this 
view  we  have  direct  evidence  that  the  fins  of  modern  sharks 
do  shorten  up  at  the  base  during  ontogeny,  and  we  know  also 
that  the  fins  of  the  oldest  fossil  sharks  (Pleuropterygidae, 
Acanthodidse,  and  Diplacanthidae)  were  of  the  fin-fold  charac- 
ter, broadest  at  the  base  and  without  any  posterior  indentation 
or  notch  such  as  modern  selachian  fins  possess. 

III.  The  facts  of  the  degeneration  of  the  most  anterior 
muscle-buds,  and  of  the  backward  extension  of  those  buds 
immediately  posterior  to  these  to  enter  the  fin,  can  not  be 
accepted  as  proof  of  the  migration  of  the  fin,  as  has  been  so 
strongly  urged  by  the  gill-arch  theorists  (Braus  ’98),  for: 

(1)  The  same  process  occurs  at  the  posterior  border  of  the 
same  fin.  The  observations  of  Braus  (’98)  demonstrate  that 
in  Spinax  the  last  pelvic  muscle-bud  degenerates  without  enter- 
ing the  fin,  while  the  buds  immediately  in  front  of  it  are  com- 
pelled to  reach  forward  to  enter.  My  preparations  of  Spinax 
show  that  four  such  buds  extend  forward  to  attain  their  posi- 
tions in  the  fin,  and  the  same  condition  is  observed  in 
Cestracion. 

(2)  But,  most  significant,  the  same  process,  I find,  occurs 
in  the  unpaired  fins.  In  the  dorsal  fins  of  Cestracion  the  most 
anterior  buds  extend  backward  while  the  most  posterior  reach 
forward  to  enter  the  fin.  Again,  in  Paul  Mayer’s  work  (’86), 
published  more  than  twenty  years  before  this  objection  to  the 
fin-fold  theory  was  raised,  we  learn  from  his  description  of 
the  unpaired  fins  of  Scyllium  and  Pristiurus  that  there  are 
abortive  muscle-buds  both  before  and  behind  the  dorsal  fins, 
and  we  may  observe  from  his  plates  that  the  most  posterior 
and  most  anterior  buds  which  enter  the  fins  have  to  reach  out 
of  position  to  accomplish  it. 

(3)  The  fact  that  in  the  pelvic  fin  these  most  anterior  muscle- 
buds  are  the  earliest  to  appear  is  not  to  be  taken  as  a proof 
that  they  are  phylogenetically  older  than  those  which  appear 
later,  as  the  gill-arch  theorists  assume  (Braus  ’98),  but  rather, 


THE  ORIGIN  OF  VERTEBRATE  LIMBS 


9 


they  appear  first  because  they  are  most  anterior,  for  it  is  in  the 
nature  of  .all  such  serial  structures  to  develop  from  anterior  to 
posterior  (e.  g.,  gills,  somites,  pronephric  tubules,  etc.)  This 
condition  is  observed  in  the  unpaired  fins  as  well  as  in  the 
paired,  for  in  the  dorsals  of  Cestracion  the  'most  anterior  buds 
are  the  first  to  develop.  The  same  principle,  according  to  my 
observations,  holds  also  among  the  various  fins,  the  pectoral- 
preceding  the  pelvic,  the  first  dorsal  preceding  the  second,  in 
time  of  development,  etc.,  yet  this  cannot  be  considered  a proof 
that  the  first  dorsal  is  phylogenetically  older  than  the  second 
nor  the  pectoral  older  than  the  pelvic.  (According  to  the  gill- 
arch  theory  the  pelvic  should  be  the  older,  yet  it  develops 
later  than  the  pectoral.) 

These  facts  are  readily  interpreted  on  the  hypothesis  that 
the  bases  of  the  fins  once  extended  over  a larger  number  of  body 
segments  than  at  present.  In  the  pelvic  it  is  evident  that  the 
shortening  has  been  much  greater  at  the  anterior  than  at  the 
posterior  edge  of  the  fin.  As  a result  the  present  fin  is  now 
situated  in  advance  of  what  was  once  its  posterior  limit,  though 
the  anterior  edge  of  the  present  fin  is  much  farther  back  than 
formerly.  Certainly  this  is  not  migration,  but  a concentration 
of  the  fin  basis  in  a manner  similar  to  that  occurring  in  the 
unpaired  fins. 

C.  If  the  six  objections  given  under  this  heading  can  be 
answered,  the  paired  fins  should  be  compared  rather  than  con- 
trasted with  the  unpaired  fins. 

I.  If  it  were  true,  as  the  opponents  of.  the  fin-fold  theory 
maintain,  that  the  skeleton  of  the  unpaired  fins  consists  of 
modified  spinous  and  haemal  propesses,  then  evidently  the  skele- 
ton of  the  paired  fins  could  not  have  a similar  origin  to  that 
of  the  unpaired.  Opposed  to  such  a view,  however,  are  the 
following  facts: 

(i)  In  the  first  and  second  dorsals,  the  anal,  and  the  superior 
caudal  fins  there  is  not  the  least  indication  of  such  a close 
relation  of  the  rays  to  the  vertebral  processes  as  we  should 
expect  from  the  above  view  of  their  origin.  In  the  lowest 
sharks,  especially  in  such  forms  as  do  not  possess  fin  spines, 
the  rays  of  the  dorsal  and  anal  fins  are  usually  widely  separated 
from  the  axial  skeleton,  according  to  the  present  studies  on 


10 


OSBURN 


many  species.  In  the  ancient  fossil  shark  Cladoselache,  also, 
the  rays  of  the  dorsal  fin  are  widely  separated  from  the  vertebral 
column. 

(2)  In  ontogeny  the  skeleton  of  the  unpaired  fin  is  plainly 
developed  from  a plate  of  thickened  mesenchyme  which  first 
appears  next  to  the  ectoderm. 

(3)  During  early  development  the  unpaired  fin  skeleton  is 
never  in  contact  with  the  axial  skeleton,  as  shown  by  my  studies 
of  Cestracion,  Chlamydoselachus,  Spinax,  Mustelus,  and  others. 

(4)  The  only  exception  to  the  last  statement,  and  the  only 
case  where  corresponding  rays  are  known  to  come  into  contact 
with  vertebral  processes,  are  found  in  the  inferior  caudal  fin, 
and  even  here  there  are  frequent  discrepancies.  This  fin  is 
then  an  exception  to  the  rule,  and  if  all  the  unpaired  fins  have 
had  a similar  origin,  as  seems  probable,  we  must  explain  the 
condition  in  this  fin  as  due  to  secondary  fusion  of  the  rays 
with  haemal  spines  to  secure  better  support.  On  account  of 
their  mechanical  relation  to  the  ventral  lobe  of  the  caudal  fin, 
which  is  the  chief  organ  of  propulsion,  these  rays  stand  in  need 
of  just  such  support.  (This  is  the  part  of  the  fin  which  becomes 
the  functional  caudal  in  Teleosts.)  Examples  of  parallel  cases 
are  the  dorsal  spines  of  sharks  (undoubted  secondary  struc- 
tures), which,  in  order  to  secure  firmer  support,  have  become 
secondarily  attached  to  the  axial  skeleton,  and  the  superior 
rays  of  the  secondarily  diphy cereal  tail  of  Dipnoi. 

II.  With  regard  to  the  comparison  of  the  girdles  of  the 
paired  fins  with  the  basalia  of  the  unpaired  fins  a number  of 
facts  present  themselves. 

(1)  The  girdles  exhibit  so  much  variation  in  form  that  they 
show  themselves  to  be  adaptive  structures  such  as  the  basalia  of 
unpaired  fins  are  admitted  to  be. 

(2)  In  the  fossil  Cladoselache  we  have  the  evidence  that 
the  pelvic  girdle  was  formed  in  the  same  way  as  the  basalia  of 
unpaired  fins, — indeed  it  is  in  the  same  condition  as  many  of 
the  unpaired  fins  of  modern  sharks. 

(3)  In  the  Notidanidae,  which  are  without  doubt  the  lowest 
and  most  primitive  of  recent  sharks,  the  pelvic  girdle  is  merely 
a flat  plate,  not  more  complicated  in  form  than  the  basalia  of 
many  unpaired  fins  and  in  Chlamydoselachus  twelve  of  the 


THE  ORIGIN  OF  VERTEBRATE  LIMBS 


11 


twenty-five  rays  of  the  pelvic  fin  attach  directly  to  the  girdle, 
thus  indicating  its  primitive  position  in  the  category  of  basalia. 

(4)  In  the  more  primitive  Ganoids  the  skeleton  of  the  paired 
fins  has  a close  resemblance  to  that  of  the  unpaired,  as  Thacher 
and  Mivart  demonstrated  long  ago.  Regan  (’04)  has  recently 
brought  forward  a remarkably  clear  case  in  Psephurus  gladius, 
in  which  the  series  formed  by  the  anal,  pelvic,  and  pectoral  is 
most  convincing.  The  pelvic  resembles  the  anal  even  more 
than  it  does  the  pectoral. 

III.  The  presence  of  post-axial  rays  in  the  pectoral  fins  of 
the  fossil  Pleuracanthus,  and  to  a limited  extent  in  modern 
selachians,  is  held  by  the  gill-arch  theorists  to  prove  the  “primi- 
tive biseriality”  of  the  paired  fin  skeleton.  Opposed  to  this 
conclusion  we  have  the  facts: 

(1)  The  fossil  Pleuropterygidae,  Acanthodidse,  and  Diplacan- 
thidae,  all  of  which  occur  in  older  strata  than  does  Pleuracan- 
thus, have  the  fins  all  decidedly  of  the  fin-fold  type,  with  not 
even  any  opportunity  for  the  presence  of  post-axial  rays. 

(2)  Post-axial  rays  are  entirely  absent  from  the  pelvic  fins 
of  all  sharks,  modern  and  ancient  (unless  we  are  to  accept 
the  questionable  cartilages  in  the  mixipterygium  of  Pleuracan- 
thus, the  homology  of  which  with  post-axial  fin-rays  is  at  least 
doubtful) 

(3)  The  occurrence  of  post-axial  rays  in  the  pectoral  fins  of 
recent  sharks  is  so  sporadic  and  variable,  and  they  are  wanting 
entirely  in  so  many  species,  that  they  are  better  considered  as 
adaptive  structures  without  peculiar  phylogenetic  significance. 

(4)  There  are  known  cases  of  post-axial  rays  in  the  unpaired 
fins  (dorsal  of  Raja,  anal  of  Heptanchus;  also  the  dorsal  of  the 
Devonian  Ganoid,  Coelacanthus) , where  they  have  all  the  ap- 
pearance of  those  in  the  pectoral.  This  again  shows  their 
adaptive  nature. 

(5)  In  higher  vertebrates  which  have  taken  up  aquatic  life  ■ 
we  have  well-known  examples  of  extra  or  supernumerary  digits 
formed  adaptively  on  the  post-axial  side  of  the  limb.  This 
in  the  whales  is  proved  to  have  taken  place  by  a longitudinal 
division  of  the  fifth  digit  (Kukenthal  ’88  and  Symington  ’06). 
A migration  of  the  fifth  digit  into  a well-marked  post-axial 
position  for  adaptive  purposes  is  also  well  illustrated  in  certain 


UNIVERSITY  OF 
ILLINOIS  LIBRARY 
AT  UR8ANA-CHAMPAJGN 


12 


OSBURN 


aquatic  reptiles  and  mammals,  as  I have  elsewhere  indicated 
(Osburn  ’06) 

IV.  Kerr  (’99)  pointed  out  that  the  real  stumbling-block 
for  those  who  have  found  themselves  unable  to  accept  the  fin- 
fold  theory  lies  in  the  fact  that  tlje  fin-rays  do  not  arise  separately 
and  later  become  fused  to  form  the  basalia.  However,  when 
we  examine  the  mode  of  formation  of  the  fin  skeletal  this  diffi- 
culty disappears.  It  must  be  noted  that  the  skeletal  structures 
of  the  fin  are  the  last  of  all  to  develop,  and  that  the  fin  already 
has  approximately  its  permanent  shape  when  the  skeleton  begins 
to  take  form.  It  consequently  makes  its  appearance  as  an 
adapted  structure,  suited  to  the  mechanical  needs  of  the  fin 
at  the  time  it  develops.  In  the  unpaired  fins,  which  the  gill-arch 
theorists  consider  primitively  metameric,  the  process  is  precisely 
similar  to  that  in  the  paired  fins.  Wherever,  in  the  unpaired 
fins,  the  rays  have  become  joined  to  form  basalia,  these  basalia 
are  present  from  the  first,  according  to  the  writer’s  studies, 
just  as  they  are  in  the  paired  fins,  and  they  are  not  formed  by 
the  fusion  of  separate  rays.  There  is,  then,  no  more  difficulty 
in  accepting  the  origin  of  the  paired  fin  basalia  from  rays  than 
there  is  in  accepting  such  an  origin  for  the  unpaired  fins, 
since  both  proceed  exactly  alike.  In  neither  case  is  there  any 
fusion  of  once  discrete  rays  to  form  basalia,  but  in  both  the 
basalia  are  adaptively  formed  as  such  from  the  first,  due  to  the 
failure  of  the  mesenchyme  to  differentiate  into  smaller  elements. 

V.  The  opponents  of  the  fin-fold  theory  have  lately  insisted 
(Fiirbringer  ’02,  Braus  ’04)  that  the  fusion  of  the  muscle-buds 
described  by  Mollier  (’93)  in  the  paired  fins  proves  the  original 
dysmetamery  of  the  paired  fin  skeleton,  since  the  fusion  to 
form  the  “musculi  radiales”  takes  place  before  the  appearance 
of  the  rays. 

The  writer’s  investigations  on  Cestracion  show  conclusively 
that  such  an  argument  cannot  be  considered  valid,  for  the  reason 
that  fusion  occurs  also  in  all  the  unpaired  fins,  which  are  held  by 
the  gill-arch  theorists  to  be  strictly  metameric.  I have  carefully 
traced  the  process  from  its  inception  and  compared  it  with  the 
same  process  in  the  paired  fins  and  there  is  no  observable  dif- 
ference. On  the  other  hand,  it  only  shows  more  clearly  the 
close  relation  of  the  paired  and  unpaired  fins. 


THE  ORIGIN  OF  VERTEBRATE  LIMBS 


13 


VI.  The  discordance  or  discrepancy  between  muscles  and 
rays  which  has  also  been  strongly  urged  as  proof  of  the  primitive 
dysmetamery  and  independent  origin  of  these  structures  in  the 
paired  fins  (Braus  ’04)  has  likewise  no  place  in  argument  against 
the  fin-fold  theory-,  for  again  tlje  same  condition  is  found  to 
occur  in  the  unpaired  fins.  My  reconstructions  of  the  first  and 
second  dorsal  fins  of  Cestracion  show  the  same  sort  of  discrepancy 
that  has  been  proved  to  exist  in  the  paired  fins. 

In  the  foregoing  pages  the  objections  to  the  fin-fold  theory 
have  been  considered;  we  may  now  mention  the  following  ob- 
jections to  the  gill-arch  theory: 

I.  The  indications  are  that  the  primitive  fin  possessed  a 
far  greater  number  of  rays  than  the  primitive  gill. 

II.  There  has  never  been  discovered  any  indication  of  an 
intermediate  stage  representing  a transition  from  the  gill  to  the 
fin. 

III.  My  observations  indicate  that  the  paired  fin  girdles 
have  not  been  abstracted  from  the  branchial  region  by  the 
spinal  muscles  in  the  manner  assumed  by  the  gill-arch  theorists. 

IV.  The  gill-arch  theory  violates  important  time  and  place 
relations. 

To  further  explain  these  statements: 

I.  The  gill-arch  theorists  have  tried  to  show  that  the  gill 
rays  have  degenerated,  and  they  reason  that  at  one  time  the 
gill  might  have  had  rays  enough  to  equal  those  of  the  primitive 
fin.  The  only  evidence  of  degeneration  thus  far  produced  has 
been  in  the  hyoid  arch,  the  rays  of  which  are  reduced  somewhat 
in  number,  but  that  is  no  indication  that  the  number  of  rays 
in  the  true  gills  has  been  diminished  by  more  than  one-half. 
Certainly  in  Cestracion  there  is  no  evidence  of  any  reduction  in 
the  number  of  rays  in  the  true  gills. 

II.  It  would  seem  that,  if  the  paired  limb  had  been  derived 
in  the  way  the  gill-arch  theorists  maintain,  there  should  remain 
in  ontogeny  some  indication  of  the  intermediate  steps  through 
which  the  gill  passed  while  becoming  a fin.  That  no  such  steps 
are  known  to  occur  in  embryology,  palaeontology,  or  comparative 
anatomy  is  a very  forcible  argument  against  such  an  origin  of 
the  fins. 


14 


OSBURN 


Fusions  of  gill  rays  resembling  somewhat  by  their  branching 
structure  the  basalia  and  rays  of  a fin  have  been  described 
in  sharks  (Braus  ’04).  However,  the  only  cases  of  this  kind 
thus  far  made  known  have  occurred  in  the  hyoid  arch,  and  even 
the  most  sanguine  adherent  of  the  gill-arch  theory  would  scarcely 
maintain  that  the  hyoid  is  progressing  in  the  direction  of  be- 
coming a fin. 

III.  Because  both  the  spinal  and  the  visceral  (trapezius) 
muscles  attach  to  the  dorsal  end  of  the  pectoral  arch,  we  are 
hardly  justified  in  supposing  that  the  one  is  abstracting  it  from 
the  other.-  Yet  on  this  basis  the  gill-arch  theorists  assume  that 
the  anterior  spinal  muscles  not  only  deprived  the  branchial 
region  of  the  pelvic  arch  but  passed  it  over  to  their  neighbors 
and  proceeded  to  abstract  another.  Why  this  kleptomania 
should  have  been  satiated  with  two  arches,  while  half  a dozen 
yet  remained,  does  not  appear. 

If  the  above  assumption  were  true,  it  might  indeed  make  a 
strong  argument  for  the  gill-arch  origin  of  the  paired  limbs,  but 
that  it  is  without  foundation -appears  in  the  light  of  the  following 
facts  in  selachian  embryology: 

(1)  As  we  have  shown,  the  pectoral  girdle  is  differentiated 
from  a thickening  of  mesenchyme  cells  which  grows  inward 
from  the  region  of  the  epidermis. 

(2)  According  to  my  observations  on  Cestracion  and  Spinax, 
the  first  anlage  of  the  pectoral  fin  is  situated  entirely  ventral 
to  the  place  of  origin  of  the  trapezius  muscle,  and  it  is  by  the 
later  growth  of  both  these  structures  that  they  finally  come 
into  contact.  The  connection  must,  therefore,  be  interpreted 
as  secondary. 

(3)  The  pectoral  girdle  of  Cestracion  as  it  develops  moves 
toward  the  gill  region.  W'hen  it  first  appears,  the  scapular  por- 
tion of  the  girdle  is  separated  from  the  last  gill  arch  by  a con- 
siderable space,  but  as  development  proceeds  the  girdle  and  the 
arch  approach  each  other  until  the  intervening  space  is  elimi- 
nated. At  first  this  space  is  fully  twice  as  great  as  that  between 
the  gill  arches;  at  35  mm.  it  is  half  passed  over,  and  at  60  mm. 
the  arch  and  girdle  are  practically  in  contact.  In  the  adult 
they  overlap  slightly. 

There  is,  moreover,  no  evidence  that  gill  arches  may  be 


THE  ORIGIN  OF  VERTEBRATE  LIMBS  15 

crowded  or  pushed  out  of  the  branchial  region.  It  has,  on  the 
other  hand,  been  proved  that  the  sixth  gill  of  Cestracion  degene- 
rated in  situ , if  the  structures  which  Mrs.  Hawkes  (’05)  describes 
are  to  be  interpreted  as  the  vestiges  of  a gill  arch. 

IV.  First,  as  to  time  relations,  it  is  important  to  note  that 
all  the  other  structures  of  the  fin  make  their  appearance  in  ad- 
vance of  the  skeleton,  and  before  the  gill  arches  are  differentiated. 
On  the  assumption  that  the  fin  has  been  formed  out  of  a displaced 
gill,  we  should  expect  to  find  the  skeleton  developing  in  an 
outward  direction  and  carrying  with  it  the  other  structures 
which  form  the  -fin.  But  instead  of  this,  the  fin  fold,  with  its 
muscle-buds,  nerves,  and  blood-vessels,  as,  well  as  the  primitive 
support  of  the  fin  (the  mesenchyme  thickening) , is  well  developed 
before  the  skeleton  becomes  evident.  Moreover,  the  fin  skeleton 
does  not  grow  into  the  fin  fold,  for  there  is  no  disarrangement 
or  shifting  of  parts  as  the  skeleton  appears,  but  the  skeleton 
forms  in  situ  by  differentiation  of  the  original  mesenchyme 
support.  This  is  just  as  it  should  be  on  the  fin-fold  theory, 
but  exactly  opposite  to  what  would  be  expected  on  the  gill-arch 
theory,  if  time  relations  stand  for  anything.  The  order  of  ap- 
pearance, furthermore,  is  precisely  as  it  is  in  the  unpaired  fins. 

Second,  with  regard  to  place  relations,  we  have  already  shown 
that  the  first  rudiment  of  the  pectoral  arch  is  more  ventral  than 
that  of  the  gill  arch,  that  it  is  more  external,  and  that  in  Ces- 
tracion at  least  it  grows  toward  the  gill  region  as  it  develops. 

A number  of  arguments  in  favor  of  the  fin-fold  theory  yet 
remain,  and  may  be  summarized  as  follows: 

I.  All  fins  of  sharks  arise  as  longitudinal  folds  of  the  epi- 
dermis. 

II.  The  muscle-buds  which  give  rise  to  the  muscles  of  the 
fins  originate  exactly  alike  in  both  kinds  of  fins. 

III.  The  nerves  which  supply  the  paired  fins  take  their 
origin  in  the  same  way  as  those  of  the  unpaired. 

IV.  The  origin  of  the  blood  supply  is  alike  in  both  kinds  of 
fins. 

V.  The  earliest  support  of  the  fins,  a plate  of  thickened 
mesenchyme,  is  of  the  same  character  in  paired  and  unpaired 
fins  and  arises  in  the  same  manner  in  both. 


16 


OSBURN 


VI.  The  time,  place,  and  manner  of  differentiation  of  the 
fin  skeleton  is  similar  in  all  the  fins. 

VII.  The  later  growth  of  the  fin  fold  and  the  constriction 
of  the  fin  at  its  base  are  similar  in  paired  and  unpaired  fins. 

VIII.  Fusions  of  rays,  basalia,  etc.,  occur  sporadically  as 
well  as  regularly  in  the  fins  of  both  categories. 

IX.  Fin  spines  are  known  in  both  kinds  of  fins. 

X.  Ceratotrichia  or  horny  dermal  rays  are  present  in  all 
the  fins  of  sharks. 

Examining  the  above  arguments  in  the  order  given: 

I.  Whether  or  not  the  longitudinal  folds  which  give  rise  to 
the  fins  have  once  been  entirely  continuous  is  a matter  of  no 
great  consequence, — though  it  seems  entirely  possible  from  the 
evidence  at  hand  that  the  fins  may  have  been  connected  in  their 
early  history.  Be  that  as  it  may  there  is  no  disputing  the  fact 
that  all  the  fins  of  sharks,  and  indeed  of  practically  all  fishes 
(the  only  known  exceptions  are  very  rare  and  embrace  forms 
of  highly  specialized  development,  e.  g.,  Lepidosiren,  Gambusia), 
originate  as  folds  of  skin.  It  must  be  noted  that  these  folds 
are  always  longitudinal.  The  gill  membrane,  which  may  be 
considered  the  homolog  of  the  fin  according  to  the  gill-arch 
theory  (since  it  contains  the  rays  of  the  gill) , is,  on  the  other  hand, 
vertical  in  origin.  If  time  and*  place  relations  have  any  meaning 
in  embryology  we  cannot  avoid  the  conclusion  that  the  most 
primitive  ancestral  fin  was  a fin  fold.  Add  to  this  the  evidence 
from  the  oldest  fossil  sharks  (Pleuropterygidae,  Acanthodidae, 
and  Diplacanthidae)  and  we  have  a clear  case. 

II.  The  muscle-buds  supplying  the  fins  on  the  dorsal  side 
of  the  body  arise  from  the  dorsal  ends  of  the  myotomes,  while 
those  supplying  the  fins  on  the  ventral  side,  paired  and  unpaired 
alike,  arise  in  exactly  the  same  manner  from  the  ventral  ends  of 
the  myotomes.  There  is  nothing  in  this  process  to  indicate 
otherwise  than  that  the  fins  have  arisen  in  situ  as  outgrowths 
from  the  body  wall. 

III.  The  fins  on  the  dorsal  side  of  the  body  are  supplied  by 
branches  of  the  dorsal  rami  of  the  spinal  nerves,  the  same  in 
character  as  those  which  supply  the  adjoining  parts  of  the  body, 
while  on  the  ventral  side,  paired  and  unpaired  fins  alike,  are 
similarly  supplied  with  branches  of  the  ventral  rami. 


THE  ORIGIN  OF  VERTEBRATE  LIMBS 


17 


IV.  In  the  embryology  of  Cestracion  I have  carefully  fol- 
lowed the  development  of  the  blood  supply  in  both  kinds  of 
fins.  In  every  case  the  blood-vessels  are  those  which  also  supply 
the  adjoining  body  wall,  and  which  take  their  origin  as  dorsal 
branches  of  the  dorsal  aorta,  or,  in  other  words,  are  typical 
body-wall  blood-vessels. 

V.  The  earliest  support  of  the  fins,  paired  and  median  alike, 
is,  as  we  have  already  stated,  a dense  or  thickened  mesenchyme. 
The  thickening  in  all  cases  begins  next  to  the  ectoderm  and 
becomes  noticeable  immediately  after  the  fin  fold  makes  its 
appearance.  As  growth  progresses  this  denser  mesenchyme  ex- 
tends inward  until  it  occupies  all  of  the  region  in  which,  later  on, 
the  fin  skeleton  is  formed.  The  procartilage  is  differentiated 
right  in  place  out  of  the  mesenchyme  support,  and  later  becomes 
chondrified  to  form  the  cartilaginous  skeleton.  This  sequence 
in  development  is  just  what  the  fin-fold  theory  would  lead  us 
to  expect,  and  it  seems  altogether  probable  that  such  has  been 
the  phylogenetic  history  of  the  supporting  structures  of  the 
fins. 

VI.  In  Cestracion  the  first  indication  of  the  formation  of 
the  skeleton  is  seen  in  the  region  near  the  middle  of  the  fin, 
and  the  bases  of  the  rays  and  the  adjoining  portions  of  basalia 
appear  at  the  same  time  and  ace  the  first  structures  to  be  ob- 
served. The  differentiation  spreads  in  all  directions,  and  the 
girdles,  in  the  case  of  the  paired  fins,  appear  very  rapidly.  In 
the  unpaired  fins  the  process  is  identical,  except  for  the  girdles. 
We  must  insist  upon  the  fact  that  the  rays  do  not  “grow  out” 
of  the  basalia  but  that  both  structures  are  differentiated  in  the 
same  way  out  of  the  same  mesenchyme  plate.  Similarly  the 
girdles  do  not  grow  out  of  the  basalia,  nor  vice  versa. 

VII.  As  we  have  seen,  the  fin  fold  originates  in  precisely 
the  same  way  in  paired  and  unpaired  fins.  The  process  of 
further  development  is  also  similar  in  both.  The  base  of  the  fin 
grows  relatively  slowly,  while  the  body  is  elongating  and  the 
external  part  of  the  fin  fold  is  pushing  out  very  rapidly.  This 
results  in  a concentration  of  the  fin  base,  in  the  manner  described 
by  Mollier  (’93)  for  the  paired  fins.  The  median  fins  pursue  a 
similar  course,  though  usually  the  process  is  not  carried  to  such 
a degree.  The  shape  taken  by  the  fins  during  development 


18 


OSBURN 


depends  upon  what  Mollier  has  termed  their  “direction  of 
growth.”  The  amount  of  constriction  at  the  base  of  the  fin 
is  presumably  measured  by  the  amount  of  mobility  required 
of  the  fin. 

VIII.  The  so-called  “fusions”  of  rays  and  basalia,  which, 
in  reality,  are  merely  failures  to  differentiate  separately  out  of 
the  mesenchyme  and  are  not  due  to  the  growing  together  of 
parts,  may  occur  at  any  part  of  the  fin  skeleton,  but  according 
to  my  observations  made  on  many  preparations  as  well  as  on 
data  provided  by  the  plates  of  various  authors,  they  occur 
much  more  commonly  at  or  near  the  ends  of  the  series  of  rays. 
It  would  seem  that  mechanical  conditions  would  naturally  be 
most  effective  in  producing  them  here.  The  mode  of  formation 
of  these  “fusions”  and  the  manner  of  their  occurrence,  both  of 
which  are  similar  in  all  fins,  lead  us  to  conclude  that  they  are 
of  one  kind  with  the  larger  basalia  and  that  all  such  sporadic  as 
well  as  regular  cases  are  produced  in  adaptation,  i.  e.  to  meet 
the  mechanical  needs  of  the  particular  fin  in  which  such  struct- 
ures occur. 

IX.  While  spines  are  not  found  in  the  paired  fins  of  modern 
sharks,  they  are  to  be  found  in  those  of  some  of  the  oldest 
fossil  forms  (Diplacanthidae  and  Acanthodidae,  also  Gyracan- 
thus,  Haplacanthus,  and  Heteracanthus).  This  shows  the  very 
similar  nature  and  potentiality  of  the  two  kinds  of  fins  at  a 
very  remote  period, — -Upper  Silurian  and  Devonian. 

X.  Horny  fin-rays  or  ceratotrichia  are  very  characteristic 
structures  of  the  fins  of  sharks  and  occur  nowhere  else  in  the 
body.  The  importance  of  these  structures  in  phylogeny  has 
recently  been  discussed  by  Goodrich  (’04)  who  finds  them  to  be 
very  ancient  and  very  conservative.  They  occur  alike  in  the 
paired  and  unpaired  fins  of  all  sharks,  even  the  most  ancient. 
(Goodrich’s  failure  to  find  the  ceratotrichia  in  the  paired  fins 
of  Cladoselache  can  be  attributed  only  to  insufficient  material, 
for  an  examination  of  the  many  specimens  in  the  American 
Museum  of  Natural  History  proves  their  presence  beyond  a 
doubt.)  As  these  structures  occur  equally  in  every  respect  in 
all  the  fins,  and  develop  in  the  same  way  and  at  the  same  relative 
time,  they  may  be  taken  to  indicate  a community  of  origin  for 
all  of  the  fins. 


THE  ORIGIN  OF  VERTEBRATE  LIMBS 


19 


When  we  consider  the  facts  derived  from  embryology,  anat- 
omy, and  palaeontology  which  are  arrayed  in  the  preceding 
pages,  the  conclusion  is  borne  in  upon  us  that  the  paired  and 
unpaired  fins  are  primarily  similar  structures,  and  the  evidence 
from  the  present  investigations  is  overwhelmingly  in  favor  of 
the  origin  of  all  fins  as  local  outgrowths  from  the  body  wall. 

Columbia  University,  New  York  City, 

March  28,  1906. 

LITERATURE  REFERRED  TO. 


Balfour,  F.  M. 

’78.  Monograph  on  the  Development  of  Elasmobranch 
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Braus,  H. 

’98.  Ueber  die  Extremitaten  der  Selachier  Verh.  Anat. 
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*04.  Tatsachliches  aus  der  Entwickelung  des  Extre- 
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Bunge,  A. 

’74.  Ueber  die  Nachweisbarkeit  eines  biserialen  Archi- 
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Davidoff,  M.  von 

’79.  Beitrage  zur  vergleichenden  Anatomie  der  hinteren 
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Dean,  B. 

*02.  Biometric  Evidence  on  the  Problem  of  the  Paired 
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Dohrn,  A. 

’83-  Studien  zur  Urgeschichte  des  Wirbelthierkorpers, 
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20 


OSBURN 


Furbringer^M. 

’96.  Extremitaten  Theorie.  Festschrift  fur  Gegenbaur , 
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’02.  Morphologische  Streitfragen . Morph.  Jahrb 
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Gegenbaur,  C. 

’65.  Untersuchungen  zur  vergleichenden  Anatomie  der 
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’70.  Ueber  das  Skelet  der  Gliedmassen  der  Wirbelthiere 
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Goodrich,  E.  S. 

*04.  On  the  Dermal  Fin-rays  of  Fishes,  Lining  and 
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Hawkes,  0.  A.  M. 

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Kerr,  J.  G. 

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Kukenthal,  W. 

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Mayer,  P. 

’86.  Die  unpaaren  Flossen  der  Selachier.  Mitt.  Zool. 
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’79.  Notes  on  the  Fins  of  Elasmobranchs,  with  Con- 
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THE  ORIGIN  OF  VERTEBRATE  LIMBS 


21 


Mollier,  S. 

*93.  Die  paarigen  Extremitaten  der  Wirbelthiere.  Anat. 
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Osburn,  R.  C. 

’06.  Adaptive  Modifications  of  the  Limb  Skeleton  in 
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’00.  On  the  Formation  of  the  Pelvic  Plexus,  with 
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Symington,  J. 

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